Behavioral Genetics - Eppp Knowledge

1. Using Falconer, what is $c^2$ if $r_{MZ}=0.70$ and $r_{DZ}=0.50$?

   $c^2 = 0.30$. Calculation gives $c^2 = 2(0.50) - 0.70 = 0.30$, showing 30% of variance from shared environment.

2. What is gene-environment correlation ($rGE$) in psychopathology?

   Genotype influences exposure to particular environments. rGE occurs when genetic factors shape environmental experiences, linking genotype to environment non-randomly.

3. What is the basic logic of an adoption study for separating genetic and environmental effects?

   Compare adoptees to biological versus adoptive relatives. Adoption studies disentangle genetics from environment by assessing similarity to biological relatives (genetic) versus adoptive ones (environmental).

4. What is the Falconer estimator for heritability using twin correlations $r_{MZ}$ and $r_{DZ}$?

   $h^2 = 2\left(r_{MZ} - r_{DZ}\right)$. Falconer's formula isolates additive genetic variance by doubling the difference in correlations, assuming equal environments.

5. What is gene-environment interaction ($G\times E$) in psychopathology?

   Genetic effects on risk differ across environmental conditions (or vice versa). GxE describes how genetic predispositions interact with environmental exposures to modulate psychopathology risk differentially.

6. What is the expected genetic relatedness of half siblings?

   Approximately $0.25$ (about $25\%$ shared segregating genes). Half siblings share one parent, leading to an average of 25% shared segregating genes.

7. What does it suggest if $r_{DZ}$ is greater than half of $r_{MZ}$ for a trait?

   Shared environmental effects are likely substantial. DZ correlation exceeding half the MZ value implies environmental factors common to siblings inflate similarity beyond genetics.

8. What does it imply if MZ concordance is less than $1.0$ for a psychopathology diagnosis?

   Nonshared environment and measurement error also contribute. Since MZ twins are genetically identical, imperfect concordance reflects unique environmental influences or diagnostic inaccuracies affecting expression.

9. Using Falconer, what is $e^2$ if $r_{MZ}=0.80$?

   $e^2 = 0.20$. With $r_{MZ}=0.80$, $e^2=1-0.80=0.20$ represents variance from individual-specific factors.

10. What does the equal environments assumption (EEA) state in classical twin studies?

    MZ and DZ twins experience equally similar relevant environments. The EEA assumes no systematic environmental differences between MZ and DZ pairs to attribute greater MZ similarity to genetics.

11. What is the definition of heritability ($h^2$) in behavioral genetics?

    Proportion of phenotypic variance due to genetic variance in a population. Heritability quantifies the extent to which genetic differences explain observed trait variation within a specific population.

12. What is evocative $rGE$ (gene-environment correlation) in child psychopathology risk?

    Genetically influenced traits evoke specific responses from others. Evocative rGE happens when a child's heritable traits elicit environmental reactions that reinforce risk pathways.

13. What is the expected genetic relatedness of dizygotic (DZ) twins or full siblings?

    Approximately $0.5$ (about $50\%$ shared segregating genes). DZ twins, like full siblings, share on average half their segregating genes due to independent fertilization.

14. Which transmission pattern is suggested by higher concordance in MZ than DZ twins for a disorder?

    Genetic influence is suggested (heritability contributes to risk). MZ twins share 100% of genes while DZ share 50%, so higher MZ concordance indicates genetic contributions to the disorder's risk.

15. What is passive $rGE$ (gene-environment correlation) in family transmission of risk?

    Parents provide both genes and correlated rearing environment to the child. Passive rGE arises as parents transmit genes and create family environments correlated with those genetic risks.

16. What is the definition of incomplete penetrance in genetic risk for psychopathology?

    Not all carriers of a risk genotype express the phenotype. Incomplete penetrance occurs when genetic risk factors do not invariably lead to the disorder due to other influences.

17. What is the expected genetic relatedness of first cousins?

    Approximately $0.125$ (about $12.5\%$ shared segregating genes). First cousins share grandparents, resulting in an average of 12.5% shared segregating genes through inheritance patterns.

18. What is the expected genetic relatedness of monozygotic (MZ) twins?

    Approximately $1.0$ (about $100\%$ shared segregating genes). MZ twins develop from a single fertilized egg, resulting in nearly complete sharing of genetic material.

19. What is the Falconer estimate for shared environment: $c^2$ in terms of $r_{MZ}$ and $r_{DZ}$?

    $c^2 = 2r_{DZ} - r_{MZ}$. This formula estimates shared environmental variance by adjusting DZ correlation for expected genetic contribution.

20. What is variable expressivity in the context of genetic liability to psychopathology?

    Same genotype produces different severity or symptom patterns across people. Variable expressivity reflects how identical genotypes can manifest variably in presentation due to modifiers like environment.

21. What is the Falconer estimate for nonshared environment: $e^2$ in terms of $r_{MZ}$?

    $e^2 = 1 - r_{MZ}$. Nonshared environment is estimated as the residual variance not explained by MZ twin similarity, including unique experiences.

22. What is the definition of pleiotropy relevant to comorbidity in psychopathology?

    One gene influences multiple phenotypes or disorders. Pleiotropy explains comorbidity by a single gene affecting multiple traits, contributing to overlapping disorders.

23. What does it suggest if $r_{DZ}$ is less than half of $r_{MZ}$ for a trait?

    Nonadditive genetic effects (dominance/epistasis) may be present. DZ correlation below half the MZ suggests nonadditive genetic effects reducing expected sibling similarity.

24. What is active $rGE$ (gene-environment correlation) in the development of psychopathology?

    Individuals seek or create environments consistent with genetic tendencies. Active rGE involves individuals selecting niches that align with and amplify their genetic propensities for psychopathology.

25. Using Falconer, what is $h^2$ if $r_{MZ}=0.60$ and $r_{DZ}=0.30$?

    $h^2 = 0.60$. Applying Falconer's formula yields $h^2 = 2(0.60 - 0.30) = 0.60$, indicating 60% of variance due to additive genetics.

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